Plasma spray guns have played an important part in many spray coating operations and have aided in coating substrates with Teflon, powdered paint, and some metals. Plasma spraying has been useful because the coatings have been more uniform, easier to apply, and easier to control. Experience with coatings of various types has indicated that, if the spray technique is to be applied to many other powdered materials, increased substrate temperatures and increased coating thicknesses must be made possible. This is particularly important when the spray methods are used in mass production applications.
Some materials, such as epoxies, which have a low melting point viscosity, flow out quickly to form a liquid film and permit a fast cooling rate. Other materials, such as Ultra-High Density Polyethylene, have a high melting point viscosity and require a slower coating rate in order to assure a proper film formation.
The area covered by a moving plasma spray gun depends upon the dimensions of the spray cone and the distance it is held from the work piece. The lower the melting point of the powder deposited, the farther the gun may be held from the work piece, resulting in a larger cross-section of the spray cone on the substrate surface. On the other hand, the higher the melting point of the powder deposited, the closer the gun must be held to the work piece.
It is well known that the high temperature existing in the plasma flame is due to the ionization caused by the electrical arc inside the gun housing and the subsequent recombination of the electrons and ions in the flame itself. This ionization produces a large amount of infrared radiation, in addition to the thermal energy resulting from the motion of the ions and electrons interacting with the streaming gas. If nitrogen is used at the degree of ionization typical of the operation of plasma guns as described in U.S. Pat. Nos. 3,676,638 and 3,627,204, most of the radiant energy generated is in the infrared region.
With respect to said patents, it has been found that the rate of deposition and the quality of the coating of various plastics and other dry powder coating materials on metallic and other substrates can be markedly improved by the use of a reflector, and also by placing the powder feed tube or tubes externally of the spray gun and supported by the spray gun so that the angle of entry of the powder onto the plasma jet can be varied, depending on which material is being sprayed. As referred to hereinabove, the improved quality of the coatings relates to uniformity of coating thickness, ease of control of obtaining a specified coating thickness, and increased speed of applying such coating to a given thickness requirement. All of these quality improvements are necessary and desirable if the plasma gun is to achieve its maximum utility, particularly in mass production spray coating applications.
Accordingly, it is the primary object of the present invention to provide a new and improved plasma spray gun which includes an adjustable powder feed conduit mounted externally of the gun so that powder may be applied to the flame after it has left the gun nozzle. The invention also provides a curved reflector attached to the gun for reflecting and focussing the infrared radiation and directing it to a limited area on the substrate where most of the radiation is absorbed and transformed into heat.
Another object of the present invention is the provision of the use of adjustable powder feed tubes for applying powder to the plasma flame to insure that the powder particles will be melted but not vaporized when they are applied to the substrate work piece.
Still another feature of the invention is the use of a curved concave reflector surrounding the plasma flame after it reaches the space between the nozzle and the substrate work piece. The reflector increases the temperature of the surface of the work piece and produces a more even coating.